This invention relates to wire handling apparatus. More particularly, it relates to an apparatus which feeds precisely known amounts of wire to a bonding mechanism that bonds the wire to a printed circuit board at a rate which does not slow down the bonding process. Even more particularly, it relates to an apparatus which, when twin lead wire is being bonded, separates the two wires of the twin lead at the appropriate places before it is fed to the bonding mechanism so that each wire of the twin lead can be readily bonded.
A typical subassembly in the electronics manufacturing industry consists of integrated circuits (IC's) soldered to a printed circuit board (PCB). The PCB has printed wiring traces, typically on internal layers, which interconnect the circuits of the IC's. Quite often, it is necessary to add discrete wire between selected pads of the PCB, e.g., to tune clock signals, to complete the interconnections which could not be made with printed wire due to lack of space, to correct mistakes in the design, etc. When the amount of wire to be added becomes excessive, it is common practice to build a bonding mechanism, usually semi-automated, to speed up this process of adding discrete wires to the PCB.
In the prior art such semi-automated mechanisms consists of a controller, an X-Y table, a wire feeder, a wire bonder, e.g., a thermo-compression head, an ultra-sonic head, a laser beam, etc., and an operator. The PCB is placed on the X-Y table, and the controller then moves the X-Y table until the first pad of the PCB at which a wire is to be bonded is positioned under the wire bonder. The wire feeder then feeds the end of the wire and the operator holds it on the pad while the bonder attaches it. The controller then moves the table to position the second pad under the wire bonder that is to have the other end of the wire bonded to it. The operator again holds the wire on the pad while it is bonded and then cuts the wire at the bond on the side closest to the wire feeder. The table is moved to position the first pad of the second wire under the bonder, the operator holds the end of the wire on the pad for bonding, the X-Y table is moved to position the second pad for the second wire, and so on until all the wires have been added to the printed circuit board.
Disadvantageously, the wire feeder as above described has several inherent problems. The first problem is speed. The X-Y table can be repositioned in a relatively short time. However, while the wire itself has little mass (the diameter of the wire being bonded in usually on the order to a thousandth of an inch), the spool that holds the wire has a large mass. Hence, if the wire feeder attempts to move wire to the bonder too rapidly, the wire may break if the large mass spool can not be accelerated at the same rate. If the wire is gradually accelerated to prevent breakage, a braking mechanism must be added to the spool to stop it when the feeding stops. The typical solution to this problem in the prior art is to feed the wire at a rate which does not cause the spool to turn at a high speed. Unfortunately, this rate takes longer to feed the wire than it does to move the X-Y table, slowing down the process.
Another problem with prior art wire feeders is controlling the length of wire being fed. The wire being bonded should be long enough to provide a small amount of slack to prevent stress and to allow the strand of wire between the two pads to be moved slightly by the operator to expose other pads for subsequent bonding. However, the wire must not be too long. Otherwise, wire build up, also called "mounding", may occur. Further, in the case of clock tuning, an exact length is desired.
A wire feeder typically includes a least one pair of pinch rollers, that is, two cylinders of suitable material such as hard rubber that circumferentially touch each other along their length. One cylinder is driven while the other idles on its shaft, and the wire to be fed passes between the two cylinders. When the driven cylinder turns, friction causes the idler cylinder to also turn, which causes the wire to move. The length of wire moved is determined by the amount of rotation of the driven roller. Unfortunately, wire slippage occurs, especially during the initial movement, which causes the amount of wire being fed to be less than calculated. In the prior art, attempts are made to compensate for the slippage by feeding more wire than desired. The result is a length of wire to be bonded that is close to the desired length, but which length has an unknown error therein.
A third disadvantage of the wire feeders of the prior art is their inability to efficiently handle twin lead wire. (Twin lead wire, consisting of two insulated wires held a constant distance apart by a manufacturing process which melts or glues their insulations together, is used when a controlled impedance transmission line is required.) When twin lead wire is used, the operator must cause both wires of the twin lead to be bonded on their respective pads. Mechanisms of the prior art either leave it up to the operator to manually separate the two leads prior to bonding, or attempt to separate them before being fed to the operator.
When the twin lead is separated before being fed to the bonding position, it is typically done by a solenoid actuated blade, or pin, that is driven between the leads as they move by on a rotating cylinder or drum. Unfortunately, the rotating cylinder has some amount of eccentricity and wobble caused by the rotation about its axis. Because of the wobble and eccenricity, the splitting blade may go too deep and be dulled by striking the cylinder, not go deep enough and not split the wires, or cut into the insulation of one or both of the wires, thereby exposing the wire to a potential short after bonding.
It is therefore apparent that a need exists in the art for an apparatus to feed wire to a bonding mechanism which can feed a precise amount of wire at a speed which does not slow down the bonding process, and which can consistently split twin lead wire as required. The present invention advantageously provides such a mechanism.